1. Field of the Invention
This invention relates to an apparatus for flowing and filling liquefied inert gas. The apparatus functions to flow and to fill a liquefied inert gas, such as a liquid nitrogen, immediately before seaming of a can in order to apply internal pressure to the can.
2. Description of the Prior Art
In the past, for obtaining an internal pressure of a can formed of a soft material, a liquefied inert gas (hereinafter merely referred to as the liquefied gas) such as liquid nitrogen has been filled into a head space of a can filled with liquid immediately before seaming of the can. The internal pressure of the can varies with a difference of a filling amount of the liquefied gas such that if the filling amount is small, the internal pressure is insufficient resulting in an insufficient strength of the can. Conversely, if the filling amount is large, over-pressure results. It has been therefore required that the filling amount is always controlled to a proper value according to the seaming condition and the like.
In the past, filling of the liquefied gas into a can in a high-speed canning line has been carried out in such a manner that the liquefied gas is continuously made to flow into cans continuously conveyed by a conveyor from a liquefied gas storage tank. The filling amount is controlled by changing an opening degree of a valve. As for the method for controlling the filling amount, a method has been proposed in which an opening degree of a valve is controlled to follow the line speed (for example, Japanese Patent Application Laid-Open Nos. 146797/1983 and 166196/1983).
In the flow-amount control devices heretofore proposed, a valve rod of a needle valve is always urged by a spring in a direction of opening the valve (upward). An upper end of the valve rod is defined by a drive rod moved up and down by means of a pinion rack mechanism or an electrically-driven cylinder. The drive rod is displaced according to the line speed to thereby control the opening of the valve at the same time when the valve is opened. The opening degree of the valve with respect to the line speed is obtained by inputting into a microprocessor in advance, a conversion table which has calculated, using internal pressure of a can as a parameter, the relationship between the line speed and the flow amount of liquid nitrogen required to obtain a fixed internal pressure, and effecting arithmetic operation on the basis of the conversion table corresponding to the detection valve of the line speed. The liquefied gases from the valve, which are filled into the cans, are stored in a sintered metal container to de-energize them. Scattering of the liquefied gases outside the can is prevented to improve the yield of the liquefied gases into the can.
However, in the above-described conventional apparatus, even if the opening degree of the valve is varied to follow the speed of the line, if a flow of liquid is received by a porous container a problem arises in that the response is so slow as to make it difficult to properly control the flow amount of the liquefied gases to follow the line speed.
Furthermore, in the above-described conventional apparatus, since the control for the opening and closing of the valve and the control of the opening degree are carried out by one and the same means, it is difficult to control the opening degree of the valve properly particularly in the early stage of operation. That is, at the start of operation of a seamer, normally, rolling of a can starts after idle operation. Therefore, in the case of a high-speed operation, cans are abruptly conveyed to the flow-down position. Thus, the opening degree of the valve needs to be set while comparing the operation of opening the valve, the opening degree of the valve during the opening and the value of the opening degree of the valve obtained by the arithmetic operation from the line speed. Therefore, the control of the opening degree of the valve is late, failing to obtain an accurate opening degree of the valve. As a result, cans can be produced which are defective in internal pressure in the early stages of the operation. In the above-described prior art, the flow amount is controlled on the basis of the specific conversion table of the line speed/the open amount of valve set for high speed and for low speed. However, the proper amount of the liquefied gases filled into the cans differs with various conditions such as the amount of the contents , filling conditions therefore, and the like. It has been difficult to obtain a proper filling amount corresponding to the change in all these conditions merely by using a specific conversion table.
On the other hand, an arrangement it has been proposed wherein a device for opening and closing a flow valve and a device for controlling an opening degree of a valve are independently organized. In order to closely control the flow amount corresponding to the change in various conditions such as a filling condition for cans, a seaming condition and the like, the inclination and offset thereof are selected corresponding to the change in the conditions to obtain an optimum conversion table which meets the conditions. The flow amount is controlled on the basis thereof to thereby overcome the problems noted above (Japanese Patent Publication No. 50200/1986). In this case, selection of the speed/opening degree conversion table according to the conditions is carried out by two units, i.e., a gain setting unit for determining an inclination and an offset setting unit for moving the table in parallel. Therefore, for example, in the case where a conversion table with only an opening degree at a low speed changed is desired to be obtained, the inclination is first changed by the gain setting unit, and two setting units had to be changed. Further, to insure whether the set is correct or not, it has been necessary to switch the low-speed operation and high speed operation to confirm an opening degree. Moreover, when a flow of liquefied gases flowing from the flow valve is directly introduced into cans, scattering increases. When a porous receiving container is provided halfway in the flow in order to overcome such a scattering, there poses a problem in that the response to the line speed is deteriorated.